Touch sensors are transparent or opaque input devices for computers and other electronic systems. As the name suggests, touch sensors are activated by touch, either from a user's finger, or a stylus or some other device. Transparent touch sensors, and specifically touchscreens, are generally placed over display devices, such as cathode ray tube (CRT) monitors and liquid crystal displays, to create touch display systems. These systems are increasingly used in commercial applications such as restaurant order entry systems, industrial process control applications, interactive museum exhibits, public information kiosks, pagers, cellular phones, personal digital assistants, and video games.
The dominant touch technologies presently in use are resistive, capacitive, infrared, and acoustic technologies. Touchscreens incorporating these technologies have delivered high standards of performance at competitive prices. All are transparent devices that respond to a touch by transmitting the touch position coordinates to a host computer. An important aspect of touchscreen performance is a close correspondence between true and measured touch positions at all locations within a touch sensitive area located on the touch sensor (i.e., the touch region).
One type of resistive touchscreen, and specifically a 5-wire resistive touchscreen, e.g., the AccuTouch™ product line of Elo TouchSystems, Inc. of Fremont, Calif., has been widely accepted for many touchscreen applications. In these touchscreens, mechanical pressure from a finger or stylus causes a plastic membrane coversheet to flex and make physical contact with an underlying glass substrate. The glass substrate is coated with a resistive layer upon which voltage gradients are excited via electrode patterns that are disposed along the periphery of the substrate. Via electrical connections to the four corners of the coated glass substrate, associated electronics can sequentially excite gradients in both the X and Y directions, as described in U.S. Pat. No. 3,591,718. The underside of the coversheet has a conductive coating that provides electrical continuity between the touch location and voltage sensing electronics. Further details regarding 5-wire resistive touchscreens are found in U.S. Pat. Nos. 4,220,815, 4,661,655, 4,731,508, 4,822,957, 5,045,644, and 5,220,136.
In a typical 5-wire resistive touchscreen, an electrode pattern along each border of the substrate is operated in both a “sourcing” mode and a “non-sourcing” mode. For example, FIGS. 1 and 2 illustrate a touch screen substrate 2 in which respective X and Y excitations are produced on a touch region 4 by applying different corner voltages (in this case, 5 volts) to an electrode pattern 6 extending along the periphery 8 of the substrate 2. The arrows represent the direction of current flow across the touch region 4, and the dotted lines represent equipotential lines, i.e., lines along which the voltage is constant. For ideal linear touchscreen performance, the equipotential lines are perfectly straight lines, as suggested in FIGS. 1 and 2. Current flows perpendicular to these equipotential lines, so lines of current flow are straight when the equipotential lines are straight.
As shown in FIG. 1, an X excitation is generated by passing current through the touch region 4 injected at the right side of the border electrode pattern 6 and collected at the left side. That is, the left and right sides are in “sourcing” (or sinking) mode for the X excitation. Ideally, for X excitation, no current enters or exits the touch region 4 from the top and bottom sides. That is the top and bottom sides are “non-sourcing” for the X excitation.
As shown in FIG. 2, a Y excitation is generated by passing current through the touch region 4 injected at the top side of the border electrode pattern 6 and collected at the bottom side. That is, the top and bottom sides are in “sourcing” (or sinking) mode for the Y excitation. Ideally, for Y excitation, no current enters or exits the touch region 4 from the left and right sides. That is, the left and right sides are “non-sourcing” for the Y excitation. Electronics can obtain touch information from a 5-wire resistive touchscreen via the voltage excitation described above, as well as current injection and capacitive architectures. A 9-wire connection scheme, which provides drive and a sense line connections between the electronics and each of the four corner connection points, is also available. These and other technologies are described in U.S. patent application Ser. No. 09/705,383, which is expressly incorporated herein by reference.
One 5-wire connection touch sensor utilizes peripheral electrode patterns with discrete overlap resistors, such as those found in Elo TouchSystems' AccuTouch™ products and disclosed in U.S. Pat. No. 5,045,644, which is expressly incorporated herein by reference. In this case, parallel resistive current paths are provided through gaps within isolation lines between peripheral electrode patterns on opposite sides of the touch region. The current paths produce an undesirable ripple non-linearity in the touch region near the peripheral electrode pattern. As a result, a finger moving across a straight line in this region will experience variations in the excitation voltage and hence variations in the measured coordinate (unless otherwise corrected for). The considerable ripple adjacent to the top and bottom resistor chains limits the accuracy of measurements in this area, and thus, the size of the effective touch region is therefore reduced.
As a result, resistor chains have been designed to reduce the ripple often found at the periphery of the touch region. U.S. patent application Ser. No. 09/705,383 discloses an approach that reduces the ripple non-linearity on the sourcing sides of the touchscreen substrate by increasing the density of discrete electrical connections between the electrode border and the touch region, i.e., the number of gaps within the isolation lines are increased.
A problem arises in that while increasing the density of discrete electrical connections between the electrode border and the touch region improves linearity on the source sides, it provides more opportunities for parasitic sourcing and sinking of current on the non-sourcing sides. A higher density of electrical connections on the non-sourcing side tends to make matters worse. In particular, if there are more connections for the touch region than there are electrode voltages, then it is hard to avoid pairs of connections to the same electrode voltage and the consequent distortion of the desired linear voltage gradient. In practice, great linearity improvement on the sourcing sides is provided with a modest decrease in linearity on the non-sourcing sides. While this may appear to be quite a reasonable engineering trade-off, the marketplace is wary of anything that degrades any aspect of touchscreen performance.
This problem was conceptually addressed in U.S. patent application Ser. No. 09/705,383 by locating some of these gaps over the junctions between the electrodes, so that the effective voltage within these gaps is halfway between the voltages of adjacent electrodes. For example, FIG. 3 shows a resistor chain 48 having Z-electrodes 50 with overlapping outer and inner portions 51, 52, the inner portions 52 of adjacent electrodes being closest at junctions 54. An array of insulating regions 55 having two gaps 56 for each overlap resistor electrode 50 runs parallel to the inner portions 52. Some of the gaps 56 are over the junctions 54. As shown in the equivalent circuit of FIG. 4, this conceptually results in alternating connections being split between two adjacent electrodes 50, so that the effective voltage is halfway between the voltages of the adjacent electrodes 50, thereby decreasing the ripple on the non-sourcing sides of the touchscreen. The main current through the series resistor chain in non-sourcing mode is notated by the “I”, while a secondary current “i” flows through the junction gap region, which is conceptually equivalent to a simple resistive voltage divider circuit composed of two equal resistances. Thus, as can be seen, the gap voltage sequence is VN−1, (VN−1+VN)/2, VN, (VN+VN+1)/2, VN+1 . . . .
It has been determined, however, that the effective voltage between adjacent electrodes 50 does not divide in practice. The insulating regions 55 are typically placed very close to the series resistor chain electrodes 50 in response to marketplace demand for minimal border width. The result is a gap width that is typically much bigger than the separation from the series resistor chain electrodes 50. With such an aspect ratio, there is insufficient room for the electrode voltages VN and VN+1 to mix together and present an averaged voltage (VN+VN+1)/2 to the touchscreen. Effectively, the equipotential lines of the touch region “see” both electrode voltages. Hence, the VN and VN+1 equipotential lines tend to terminate on the electrodes 50, and all the equipotential lines in between VN and VN+1 bunch up at the junction 54, as illustrated in FIG. 5. As such, the resistor chain 48 of FIG. 3 will, in practice, have the equivalent circuit illustrated in FIG. 6.
There thus remains a need to improve the non-sourcing side linearity of touchscreens with discrete resistor structures.